Phosphors are compounds that exhibit a sustained glow (phosphorescence) in response to the absorption of an energized particle, such as an electron or a photon. The sustained glow results from the ability of a phosphor material to store energy for a period of time before re-emitting it. Phosphors are a fundamental component in countless display devices, light sources, and other devices, including safety equipment and novelty items. For example, phosphors are indispensable in producing white light or light of various colors from displays and lighting sources. There are a large number of phosphor compounds that have well established color responses and persistence, i.e., the duration of glow after excitation. Phosphors are chosen for particular applications based upon color response (emission spectrum) and persistence.
Phosphor coatings have been studied widely, and are applied in different thicknesses on the surfaces of various materials. Phosphor films have been applied to glass and other surfaces for many years. Several past efforts have mixed phosphor materials with glass or plastic, but the formation methods and resulting layers have limited application. A few patents have provided processes for preparing layers of phosphors embedded in materials such as glass. One example is U.S. Pat. No. 2,857,541 which describes a method for producing a thin layer of phosphor-embedded glass by mixing powdered glass and phosphor with an electrolyte and water to form a slurry. From the resulting green plaque, layers having a minimal thickness on the order of 2 mm are realized. Such a formation method and the resultant phosphor-embedded glass slab are not amenable to uniformly coating phosphor onto microstructured and irregular surfaces.
Roohollah S. Targhatr, et al., “Realization of Flexible Plasma Display Panels on PET Substrates,” Proceedings of the IEEE, VOL. 93, NO. 7, July 2005, proposes a flexible plasma display that has a top polyethylene terephtalate (PET) substrate with phosphor grains that are blast-embedded into the PET substrate. The blasting of phosphor particles embeds the phosphor particles into PET craters. In a variation, vertical etching is used to form craters on the top substrate via a photo-chemical reaction which yields a vertical and sharp etching of squares, and the particles are then deposited into the craters. The top PET layer with phosphor acts to convert vacuum ultraviolet (VUV) radiation into visible light.
Any process for preparing thin phosphor films of precisely controlled thickness and efficient in generating light, should account for several factors. If the purpose of the phosphor is to convert short wavelength (ultraviolet) radiation into visible light, it is important to distinguish phosphor layers photoexcited by VUV radiation (wavelengths less than approximately 200 nm) from phosphors intended to be illuminated by longer-wavelength ultraviolet light (200-400 nm, in the so-called UV A,B, and C regions). The reason for the distinction is that VUV photons are strongly absorbed by virtually all materials in which one might embed a phosphor. Consequently, it is preferable that phosphors intended for illumination by VUV light are exposed directly to the incoming radiation. Inserting most materials between the phosphor and the VUV source will result in some or most of the VUV photons being absorbed by said material and, thus, never reaching the phosphor. On the other hand, if it is intended that longer wavelength (λ≧250 nm) photons excite the phosphor, one has greater freedom in inserting a thin layer of one or more materials between the UV source and the phosphor because a greater variety of materials transmit efficiently in this range of wavelengths. In summary, the use of binders, glasses, or other materials to encapsulate or partially shield the phosphor is undesirable if the phosphor is to be “driven” by VUV photons. However, even if the intent is to illuminate the phosphor with photons having wavelengths above 200 nm, it is desirable to minimize the thickness of any encapsulating materials because the absorption (and reflection) of light is not zero for even the best materials.
Another consideration important to forming phosphor layers is that phosphors are generally large molecules that can be damaged if the method of depositing the layers is overly aggressive physically or chemically. Therefore, the blast embedding of phosphors into a surface is not desirable, and experience has shown that phosphor particle sizes in the 1-10 μm range are preferable.
Microcavity plasma devices and arrays have been developed and advanced by researchers at the University of Illinois, including inventors of this application. Devices and arrays have been fabricated in different materials, such as ceramics and semiconductors. Arrays of microcavity devices have been fabricated in thin metal and metal oxide sheets. Advantageously, microcavity plasma devices confine the plasma in cavities having microscopic dimensions and require no ballast, reflector or heavy metal housing. Microcavities in such devices can have different cross-sectional shapes, but generally confine plasma in a cavity having a characteristic dimension in the range of about 5 μm to 500 μm.
Applying uniform layers of phosphors to the surfaces of microcavity devices or other irregular surfaces is often challenging. Arrays of microcavities, in particular, often have inclined or spatially modulated surfaces with considerable microstructure that can include steps or gratings, not to mention the microcavities themselves. Applying a phosphor film to the interior surface of fluorescent light tubes has been a part of the manufacturing process for years but the surface to be coated is reasonably smooth and no effort is made to encapsulate the phosphor particles. Furthermore, the phosphor layer formation process often involves water which, if not removed completely from the phosphor in subsequent processing (baking, de-gassing), will adversely impact the performance and lifetime of the lamp.
The present invention addresses the need for a method to uniformly coat irregular and microstructured surfaces with one or more thin layers of phosphor. In addition, the individual phosphor particles can be coated by, or partially or wholly encapsulated (in a glass or other material), thereby protecting the phosphor from the microplasma or vice-versa.